Microdialysis of extracellular intracerebral human tissue in association with intracranial depth electrode electrographic recording in presurgical patients offers an exceptional opportunity to study several aspects of antiepileptic drug pharmacology. Patients undergoing intracranial recording (phase III) receive antiepileptic drugs daily with planned decreases and discontinuation during evaluation. After recording of seizures, treatment is reinstituted either orally or parenterally. Throughout the period of intracranial monitoring, microdialysis is carried out on a continuous basis along with simultaneous electrographic recording of neuronal activity from the same tissue being chemically sampled. These recordings, accompanied by blood sample analysis allow three important questions to be addressed: 1. What is the time course and magnitude of intracerebral extracellular brain antiepileptic drug concentration relative to dose, route of administration, and blood levels? 2. What pharmacodynamic effects on electrographic epileptiform activity are found due to antiepileptic drug changes? 3. What effect is observed on brain GABA concentrations with use of various antiepileptic drugs?